Introduction

A reinforced concrete slab is the one of the most important component in a building. It is a structural element of modern building. Slabs are supported on columns and beams. RCC slabs whose thickness ranges from 10 to 50 cm are most often used for the construction of floors and ceilings. Thin concrete slabs are also used for exterior paving purpose. In many domestic slabs are also used for exterior paving purpose. In many domestic and industrial building a thick concrete slab, supported on foundation or directly on the sub soil, is used to construct the ground floor of a building. In high rise buildings and skyscrapers, thinner, pre-cast concrete slabs are slung between the steel frames to form the floors and ceilings on each level.

 

Types of Design

For a suspended slab, there are a number of designs to improve the strength-to-weight ratio. In all cases the top surface remains flat, and the underside is modulated:

Corrugated Slab:  Usually where the concrete is poured into a corrugated steel tray. This improves strength and prevents the slab bending under its own weight. The corrugation run across the short dimension, from side to side.

Ribbed Slab: Giving considerable extra strength on one direction.

Waffle Slab: Giving added strength in both directions.

 

Construction

Concrete slab can be cast in two ways: It could either be prefabricated or cast in situ.

Prefabricated Concrete Slabs are cast in a factory and then transported to the site ready to be lowered into place between steel or concrete beams. They may be pre-stressed, post-stressed, or unstressed. Care should  be taken to see that the supporting structure is built to the correct dimensions to avoid trouble with the fitting of slabs over the supporting  structure.

In situ concrete slabs are built on the building site using formwork. Formwork is a box-like setup in which concrete is poured for the construction of slabs.

 

Materials used for the formwork

The formwork is commonly built from wooden planks and boards, plastic, or steel. On commercial building sites today, plastic and steel are more common as they save labour. On low-budget sites, for instant when laying a concrete garden path, wooden planks are very common. After the concrete has set the wood may be removed, or left there permanently. In some cases formwork is not necessary- for instance, a ground slab surrounded by brick or block foundation walls, where the walls act as the sides of the tray and hardcore acts as the base.

 

Span- Effective Depth Ratios

Excessive deflections of slabs will cause damage to the ceiling, floor finishes and other architectural details. To avoid this, limits are set on the span-depth ratios. These limits are exactly the same as those for beams. As a slab is usually a slender member the restriction on the span-depth ratio becomes more important and this can often control the depth of slab required in terms of the span-effective depth ratio is given by,

Minimum effective depth = span/ (basic ratio x modification factor)

 

Process

Shuttering

The process of constructing the RCC slab commences by erecting the centring and shuttering. Wooden shuttering is usually provided for the purpose although steel shuttering is recommended for getting a good under surface of the slab. Once the shuttering has been laid in a level, it has to be cleaned properly.

Reinforcement

The reinforcement bars are then laid as per design in both directions and tied properly with binding wire.

Mixing Concrete

All raw materials, cement, sand, coarse aggregates and water should be measured as per requirement of mix design.

 

Note: The raw materials are then mixed uniformly, while ensuring that a proper water-cement ratio is maintained which is just enough to impart sufficient workability needed to pour concrete. Ideally, the mixing should be done in a mechanical mixer. The concrete mixer should be allowed to rotate for at least 2 minutes.

Pouring Concrete

The concrete is then laid continuously to the required thickness and compacted using a needle vibrator to ensure that the cement slurry adequately fills the gaps between aggregates. Vibrated effectively such that no air gaps are left in the concrete. After the concrete has been poured, it should be finished with a steel float to ensure that the top surface is finished smooth and to make the adjustments for uniform slab thickness.

Curing

Curing is critical for a good quality concrete slab and it is commonly done by flooding the slab with water after dividing the slab into smaller portions with cement mortar partitions. The curing of slab should begin the next day of casting and continue for at least 14 days for adequate concrete strength. Inadequate curing will result in poor quality concrete and can undo the benefit of an appropriate mix and good quality mixing and compaction.

 

Advantages

• Easy to construct, the raw materials are easily available in almost all places.
• Fire resistant
• Provides permanent durable roofing, capable of multi-storied construction.
• Very versatile and can be cast in any form.
• Being a conventional option, the skill level for casting RCC slabs is commonly available.
• Requires very low maintenance.

 

Limitations

• A typical 120mm slab weight 300kg/m². This has to be designed for a live load of at least 150kg/m². This makes the self weight of the slab as 50% structural efficiency.
• The concrete in the tension zone does not take any stress and thus it is expensive to provide the same in the tension zone.
• The embodied energy of a conventional RCC slab is high because of its two main energy intensive constituents cement and steel.